Vertical milling machines are used extensively to perform a variety of machining operations, such as drilling, reaming and tapping, wherein a vertically oriented tool is removably secured in and projects downwardly from the lower end of a power driven spindle. When it is necessary to remove one tool and mount another tool in the spindle, the power must be turned off to stop the rotation of the tool. In a shop where several different machining operations are carried out on the same machine, the frequent shutting-down of the machine for tool changes is not only inconvenient, but also results in a considerable amount of lost time. It is therefore an object of the present invention to provide a drive mechanism wherein the driving connection between the tool carrier and the spindle can be interrupted so that tools can be changed while the spindle continues its rotation.
In some operations, such as drilling with a small-diameter drill bit, it is desirable to move the tool into engagement with the workpiece under controlled conditions of tool rotation in order to prevent tool breakage and the like, and it is an object of the present invention to provide a drive mechanism wherein simple hand pressure can slow down or stop the rotation of the tool when such controlled rotation is desired.